As the brain fitness craze grows, so do the number of software programs that promise to boost mental skills. Here, we take a look at three of the largest companies in the digital brain health industry. Each offers trial games that users can play free:

With more than 35,000 registered users — and more than 600 million game plays — Lumosity has a strong presence in the brain-training circuit. Users' cognitive abilities are rated on a Brain Performance Index, or BPI, based on scores from tasks designed to test memory, attention, speed, flexibility and problem-solving skills.

FOR THE RECORD: Brain games: In the June 1 Saturday section, an article about software programs that promise to boost mental agility gave the number of regular Lumosity users as 35,000. At the time of publication, the reported total was 35 million. However, Lumosity says its users have now topped 40 million.

Games such as "Word Bubbles Rising," in which you create as many words as you can from a word stem, are designed to increase verbal fluency and reduce those embarrassing "tip-of-the-tongue" moments. In "Lost in Migration," you respond to the flight direction of birds in a flock to boost your focus.

Also available as an app for Apple's mobile devices, Lumosity offers subscription options, including a $14.95 per month plan, a yearly contract for $6.70 per month or a one-time fee of $299.95.

Posit's online brain-training software, BrainHQ, features digital brain games designed to improve not only memory, attention and processing speed, but also interpersonal skills and overall intelligence. In "Visual Sweeps," players recall the direction of visual patterns that sweep across the screen — a task designed to speed up processing and reaction time.

In "Double Decision," players gaze at a Southwest landscape while paying attention to both a vehicle in the center of the screen and a road sign in the periphery. As the background becomes more complex and more distractions are added, the game is designed to expand your field of view and enhance safer navigation.

BrainHQ can be used on personal computers or the iPad; subscriptions are available for a monthly fee of $14 or an annual payment of $96.

After you log in to CogniFit, the program asks you why you want to train your brain, with answer prompts ranging from the basic "I would like general training" to the more specialized "I need to remember something" or even "I feel depressed." Based on your response, CogniFit offers personalized brain games. You can also select a task to play.

With "Words Birds," players chase letter-wielding birds across the screen as they try to spell words, in order to practice working memory and divided attention skills. In the game "Perfect Tension," players try to remove shapes from a balanced structure without causing the structure to collapse.

Beyond a handful of free introductory applications, each CogniFit application has a price set in "neurons." To get neurons, you can train and earn them — or buy them with a credit card. Each neuron is worth a penny, and the applications cost 350 neurons each. An app is available for Apple's mobile devices.